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Comment by tialaramex | Hacker News Reader

> It's perfectly valid to not always care about the starting value in a register even if you're going to use it. Plenty of pseudo-rng type code has read arbitrary registers or addresses as a source for some of their calculation without ever caring what the starting value was.

No. Never do this. If you don't care and just want random numbers use for example RDRAND, if you do care, design a proper PRNG with your preferred characteristics and seed management. YOLO programming is a bad idea always but it's especially bad when applied to assembly.

To underscore what this software is for let me quote you a diagnostic it emits:

{instruction} implicitly reads {register}, but nothing in this template produces a value for it; pin an input parameter to {register}, or write {register} before this instruction

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> No. Never do this.

I agree you shouldn't do it, that doesn't make it invalid to do.

> To underscore what this software is for let me quote you a diagnostic it emits

Well, then I stand corrected on the intent. That pretty clearly spells out that the template checker will validate that (at least for non-branching assembly) all implicitly read registers are given an explicit value and error if not. So that should mean the example in the article actually would not compile, since the rexcode definition lists both RAX and RCX as `implicit_rd`.[1] I appreciate you finding that bit of validation code and persisting in helping me learn something new, this isn't an area of coding I spend a lot of time in so it was instructive.

[1]: Does it compile? I don't have any odin dev stuff set up, but I might just try it in the next little bit just to see.

I don't run nightly Odin. I do happen to a have a non-nightly Odin installed because I was wondering if some of the trash in their kitchen sink library is in fact trash (it is) or whether it's clever in a way I didn't understand ‡

However my reading of the implementation is that:

1. Bill over-sells the value of rexcode. rexcode is Odin code, so the actual "value" derived in Odin's own compiler is just that it scrapes the data out of rexcode. That's not nothing but it's not much.

2. Bill's new template feature doesn't remember that we can pin an output, so it notices that ECX is pinned and concluded it is safe for CPUID to read it. That's a very small bug, and it's even possible I've misunderstood it, but that's my reading.

‡ Odin provides a lot of sorting algorithms. I wanted to measure how fast they are, which I did by comparing against a Rust install on the same toy machine, all of them are much slower than Rust's built-in sorts, but interestingly the provided "slice sort" which is most analogous to Rust's [T]::sort and [T]::sort_unstable is a "Smooth sort" which is relatively obscure and actually is only as slow for pre-sorted input as my Rust sorts are for unsorted input whereas most of what's provided is way slower even for pre-sorted input.

1. I don't think I am overselling rexcode, rather there isn't anything like it in the first place as a single library with that many ISAs and IRs. That's what I love about it, since I can now trivially make (non-optimizing) compilers without needing to use LLVM or use another external tool (including outputting C or assembly). Also all of those tables become binary blobs, so you could just use them in your favourite language any way.

2. That was a bug and it has now been fixed, along with many other analysis bugs.

3. The sorting algorithms are getting an overhauls soon to be a lot faster.

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